1. Field of the Invention
The present invention disclosed in the specification relates to a liquid crystal display device having pixel areas arranged in a matrix form on a same substrate, particularly to an active matrix type liquid crystal display device having semiconductor devices using semiconductor thin films. Silicon films can representatively be used as the semiconductor thin films.
2. Description of Related Art
In recent times, technologies for making a semiconductor device using semiconductor thin films, for example, a thin film transistor (TFT) on an inexpensive glass substrate have been rapidly developed. The reason is that demand for an active matrix type liquid crystal display device has been enhanced.
According to an active matrix type liquid crystal display device, TFTs are arranged to each of pixel areas of several tens through several millions arranged in a matrix and electric charges inputted to and outputted from respective pixel electrodes are controlled by the switching function of the TFTs.
Here, an explanation will be given of the basic structure of an active matrix type liquid crystal display device arranged with thin film transistors in reference to FIGS. 1(A) and 1(B). Firstly, FIG. 1(A) is a view showing a section cutting a liquid crystal display device illustrated by FIG. 1(B) in a direction orthogonal to a substrate. The section corresponds to a section cut by a broken line designated by a line A-A' of FIG. 1(B).
Numeral 101 designates a substrate having transparency on which an insulating film (not illustrated) is formed. Numeral 102 designates an active layer of a TFT, numeral 103 designates a gate electrode, numeral 104 designates a data line, numeral 105 designates a drain electrode, numeral 106 designates an interlayer insulating film, numeral 107 designates a black matrix, numeral 108 designates a pixel electrode comprising a transparent conductive film and numeral 109 designates an alignment film.
The whole substrate having TFTs which comprise as described above is hereinafter referred to as an active matrix substrate. Although attention is paid to only one pixel area according to FIG. 1(A), the active matrix substrate is actually constituted by several tens through several millions of the pixel areas and drive circuits driving the pixel areas.
Meanwhile, numeral 110 designates a substrate having transparency, numeral 111 designates an opposed electrode constituted by a transparent conductive film and numeral 112 designates an alignment film. The whole substrate comprising as described above and opposed to the active matrix substrate is referred to as an opposed substrate.
After performing rubbing treatment for regulating the alignment of a liquid crystal material in later steps, the active matrix substrate and the opposed substrate are pasted together as to be opposed to each other by a seal member, not illustrated.
In that case spacers, not illustrated, are interposed between the both substrates with a uniform density whereby a uniform substrate interval (referred to as cell gap) is obtained. Strictly speaking, in the case of the structure illustrated by FIG. 1(A), a distance between the alignment film 109 on the side of the active matrix substrate and the alignment film 112 on the opposed substrate is the cell gap.
The seal member serves not only as an adhesive agent for pasting the both substrates together also as a seal member for sealing a liquid crystal material between the both substrates at an image display region comprising a plurality of pixel areas.
Thus, a liquid crystal material 113 is sealed in an image display region (each of the plurality of pixel areas) as illustrated by FIG. 1(A). In this way, the active matrix type liquid crystal display device having the constitution as illustrated by FIG. 1(A) is formed.
According to the pixel area illustrated by FIG. 1(A), an image signal controlled by the thin film transistors is stored at a condenser formed between the pixel electrode 108 and the opposed electrode 111 with the liquid crystal material 113 as an insulating layer.
At this moment an electric field in correspondence with a voltage level of the image signal is formed between the pixel electrode 108 and the opposed electrode 111 in the case of an analog gray scale system. Further, various gray scales of image displays can be carried out by using the property of the liquid crystal material 113 where an optical response is varied in accordance with varying of the applied voltage.
A nematic group liquid crystal material (for example, TN (Twisted Nematic) type or STN (Super Twisted Nematic) type liquid crystal material) is generally used frequently as a liquid crystal material. According to the liquid crystal display device as illustrated by FIG. 1(A), the nematic group liquid crystal material is provided with a property where the long axis direction of the liquid crystal is substantially in parallel to the substrate (however, a pretilt angle may be provided) when the electric field is applied thereto and the long axis direction is directed to an electric field direction when the electric field is formed.
Accordingly, the long axis direction is varied in accordance with presence or absence of the electric field applied on the liquid crystal material. Thus the image display is carried out by controlling the amount of transmittance of light by the amount of variation of the long axis direction.
However, such a behavior of the liquid crystal material is a phenomenon applicable only when the direction of the electric field formed between the pixel electrode 108 and the opposed electrode 111 is in vertical to the substrate.
For example, in a region where a horizontal electric field substantially in parallel to the substrate is formed, the alignment of the liquid crystal material is disturbed whereby alignment defect is caused and desired image is not provided.
Normally, when a cell gap is provided as to be suitable for the applied voltage on the pixel electrode 108, a vertical electric field (electric field orthogonal to the substrate) is dominant. However, as the cell gap is increased, the influence of the vertical electric field is weakened whereas the influence of the horizontal electric field is strengthened.
Here, FIG. 1(B) is a view showing from the top face pixel areas where alignment defects of the liquid crystal material are caused by the influence of the electric field in the horizontal direction. Incidentally, areas except for the image display area is masked by the black matrix 107. Therefore, wirings and the like disposed below the black matrix are shown by dotted lines.
In FIG.(B), white lines formed in the image display areas (areas not masked by the black matrix 107) show disturbances of image display caused by alignment defects of the liquid crystal material, which are referred to as disclinations. These areas are under a state where abnormality occurs, different from the inherent alignment state of liquid crystal molecules.
As one cause of the occurrence of the disclination, firstly, influence of the horizontal electric field occurred by cross talk among wirings or among the thin film transistors, is pointed out.
For example, many of the disclinations, as illustrated at the upper stage of the pixel area of FIG. 1(B), were observed according to experiments by the inventors. It is conceived that the horizontal electric field is formed by a potential difference between the front end of the gate electrode 103 and a portion where a gate line 114 and a data line 104 intersect with each other.
This phenomenon is more manifested as the width of the pixel area (pixel pitch) is narrowed, that is, an inter-wiring distance is narrowed in pursuit of highly fine image display. Incidentally, the pixel pitch is defined by the short side of the pixel area.
Further, narrowing of the pixel pitch signifies relative enlargement of the cell gap and it is anticipated that the influence of the horizontal electric field will more be strengthened unless the cell gap is pertinently changed in accordance with the pixel pitch.
Other than the above-described case, there were cases where a disclination as illustrated at the lower stage of FIG. 1(B) was observed due to the disturbance in the alignment of the liquid crystal material in the vicinities of spacers 115 that are arranged for securing the cell gap.
Also, as other cause stepped differences of the pixel electrode 108 are caused at the surrounding of wirings or the thin film transistors. The problem is that the disclination occurs at the surrounding of the stepped difference when the alignment treatment is incomplete in the rubbing operation caused by the presence of the stepped difference or by a horizontal electric field caused at the side face of the stepped difference.
Almost all of the disclinations caused at the surroundings of the stepped differences are masked and hidden by the black matrix. However, only a contact portion for bringing the active layer 102 and the pixel electrode 108 into contact with each other, may not be masked by the black matrix 107 and the disclination may be observed there.
The disclinations formed at the pixel areas of the liquid crystal display device give an extremely unpleasant feeling visually such as blurring the display image as a whole, or the like.